Used to be the mfr's would suggest running a heavier weight oil in summertime and a lighter oil in wintertime, for appropriate startup characteristics. Now it's the same oil year round. Oils and their performance characteristics have improved greatly over time, partly reflected in the SAE ratings. So I would take the mfr's guidance seriously for year-round performance. If you consider changing to another oil/weight, compare the characteristics to make sure that your new oil will meet or exceed the requirements, the "meets xxxx specs" being a starting point.
Although this is not as much of an issue anymore, in the past, when dino spec'd oil was the norm, people started using a full synth (usually Mobil 1) of the same weight. Unfortunately, the full-synth oil did not really behave even close to what the dino oil was expected to, and proved too thin for practical use (outside engine tolerances), causing oil leaks through seals. So if you were supposed to use 5W30 dino, you would have to use 5W40 full synth for comparable performance inside the engine.
